Vanadium base alloy

ABSTRACT

SOLUTION AND PRECIPITATION HARDENED VANADIUM-BASE ALLOYS HAVING EXCELLENT RESISTANCE TO CREEP DEFORMATION AT ELEVATED TEMPERATURES AND COMPRISING, BY WEIGHT, FROM ABOUT 1% TO 4% ZIRCONIUM, FROM 3% TO 20% OF AT LEAST ONE ELEMENT SELECTED FROM A GROUP CONSISTING OF CHROMIUM, COLUMBIUM, TUNGSTEN, MOLYBDENUM, TITANIUM, IRON, NICKEL, AND TANTALUM, FROM 0.02 TO 0.1% CARBON, FROM 0.0I% TO 0.05% NITROGEN, AND OXYGEN IN AMOUNTS NOT IN EXCESS OF ABOUT 0.15%.

Aug. 3, 1971 w, POLLACK ETAL 3,597,193

VANADIUM BASE ALLOY Filed March 16. 1967 2 Sheets-Sheet l I I I z I I [I l i, m I U I C4 I U) I f, I a l F a H E A O i i i TIME HOURS CREEP CURVES OF VANADIUM ALLOYS AT 700C AND 3000O psi SPECIMENS ANNEALED I HOUR AT I200C WITNESSES INVENTORS 5 FIG. I. William Pollock and Richard T. Begiey.

BY M Q 3, 1971 w. POLLACK ET VANADIUM BASE ALLOY 2 Sheets-Sheet 2 Filed March 16. 1967 TIME- HOURS CREEP CURVES OF VANADIUM ALLOYS AT 800C AND lOOOOpsi SPECIMENS ANNEALED IHOUR AT l200C FIG. 2.

United States Patent 3,597,193 VANADIUM BASE ALLOY William Pollack, Pittsburgh, and Richard T. Begley, Bridgeville, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed Mar. 16, 1967, Ser. No. 623,697 Int. Cl. C22c 33/00 US. Cl. 75134 15 Claims ABSTRACT OF THE DISCLOSURE Solution and precipitation hardened vanadium-base alloys having excellent resistance to creep deformation at elevated temperatures and comprising, by weight, from about 1% to 4% zirconium, from 3% to 20% of at least one element selected from a group consisting of chromium, columbium, tungsten, molybdenum, titanium, iron, nickel, and tantalum, from 0.02 to 0.1% carbon, from 0.01% to 0.05% nitrogen, and oxygen in amounts not in excess of about 0.15%.

Fast breeder reactors operate at temperatures of from 550 C. to 800 C., usually about 600 C. to 700 C., with sodium metal or NAK metal circulating therethrough. The development of fast breeder reactors employing the liquid alkali metals is dependent among other things upon the availability of alloys which can withstand the rigorous environment in which they are employed; particularly capable of withstanding high operating stresses and the corrosive efifects of the liquid alkali metals at high operating temperatures of from 550 C. to 800 C. Alloys subjected to the operating conditions of a fast breeder reactor must have a combination of properties including good fabricability, strength at elevated temperatures of from 550 C. to 800 C., good resistance to creep deformation at these elevated temperatures, and corrosion resistance to the liquid alkali metals at such temperatures. It is necessary that the alloys not affect detrimentally the operation of a breeder reactor when used as cladding and structural components.

Unalloyed vanadium does not have the desired high temperature strength (700 to 800 C.) and corrosion resistance properties necessary for employment as fuel cladding for a fast breeder reactor.

While some vanadium base alloys containing reactive metal additions such as zirconium and titanium have been developed, such alloys have high creep rates and are useful primarily for short time applications at high temperatures and reasonable loads; that is, for periods of use of from to 100 hours at 700 C.

High tensile strength is not necessarily indicative of good resistance to creep deformation. Various vanadium base alloys containing 40 to 50% titanium as a major alloying component have displayed attractive tensile properties at elevated temperatures for short time applications, but'the creep rate is excessively high. Other vanadium base alloys having high creep strength properties are so heavily alloyed that the compositions for the most part are unsuitable for use as a cladding material for fast breeder reactors. There is an acute need for fuel cladding material for fast breeder reactors having the desired combination of properties of good corrosion resistance at elevated temperatures, particularly to alkali metals, high tensile strength, and good resistance to creep deforma- 3,597,193 Patented Aug. 3, 1971 tion at elevated temperatures over extended periods of time.

In accordance with this invention, it has been found that certain vanadium base alloys may be employed as cladding for fuel material for fast breeder reactors which alloys have outstanding resistance to creep deformation at 700 to 800 C. for extensive periods of time, good tensile strength at these elevated temperatures and go d fabricability.

Accordingly, it is an object of this invention to provide a vanadium base alloy having unusually good resistance to creep deformation at elevated temperatures and having highly satisfactory properties of fabricability and tensile strength at elevated temperatures.

Other objects and advantages of the invention will be obvious and appear hereinafter.

For a better understanding of the invention, reference is made to the drawings, in which:

FIG. 1 is a graph showing creep curves of typical vanadium base alloys at 700 C.; and

FIG. 2 is a graph showing creep curves of typical vanadium base alloys at 800 C.

The vanadium base alloys of this invention contain both solid solution hardeners and a dispersed second phase comprising complex carbides, nitrides, and oxides which restrict dislocation motion and grain boundary movement and thereby improve the resistance of alloys to creep deformation.

The vanadium base alloys of the present invention are composed of zirconium from about 1% to 4%, carbon in amounts of 0.02 to 0.1%, nitrogen in amounts of up to 0.05 and a solid solution hardener in amounts of from 3% to 20% of at least one of the elements selected from a group consisting of chromium, columbium, iron, nickel, molybdenum, titanium, tantalum and tungsten. More specifically stated, the vanadium-base alloys com prise, by weight, from about 1% to 4% zirconium, from 3% to 20% of at least one element selected from the group consisting of chromium, columbium, tungsten, molybdenum, iron, nickel, and tantalum. In some alloys, the zirconium content may be replaced either wholly or in part by titanium and/ or equimolar proportions of hafnium. It is preferred that chromium when present does not exceed about 12%; columbium when present does not exceed about 12%; tungsten when present does not exceed 12%, and preferably is present with a nearly equal amount of chromium; molybdenum when present does not exceed 9%, and preferably with chromium being present when the molybdenum exceeds about 3%; iron when present does not exceed 8%; nickel when present does not exceed 3% and tantalum when present does not exceed 12%. Iron gives good results when present with nickel. Carbon and nitrogen are present in amounts of from 0.02% to 0.1% C., and from 0.001% to 0.05% N. The atomic ratio of zirconium to carbon and/or nitrogen ranges from about 1.5:1 to 2:1. Oxygen is present from trace amounts to about 0.1% and, at most, not exceeding 0.15 Aluminum may be present in amounts of up to 2%. The balance is essentially vanadium, except for traces of incidental impurities such as silicon, manganese, phosphorus, etc. not exceeding a total of 0.5%.

For a reactor, the zirconium is preferably low in hafnium. For fast breeder reactors, a small trace of hafnium may be present. However, for other applications, particularly removed from the nuclear reaction zone proper,

such as piping conveying hot sodium to a heat exchanger, the zirconium may be normal zirconium With up to 3% hafnium present. Also hafnium may replace the zirconium on an atom for atom basis for high temperature application not involving nuclear reactions.

Improved creep resistance properties in the alloys of this invention are obtained by introducing the specified amounts of zirconium which combine with the recited amounts of carbon, oxygen and nitrogen to form precipitates of zirconium carbide, zirconium nitride, and zirconium oxide on precipitation hardening. Small amounts of titanium may be substituted either fully or partially for zirconium and the precipitates of titanium carbide, oxide and nitride will also form. Thus, alloys having two phases each of which contributes to the creep resistance; namely, solid solution of vanadium with one or more of the elements selected from the group consisting of columbium, chromium, tungsten, molybdenum, iron, titanium, nickel, and tantalum as substitutional solutes, and a dis- Another desirable vanadium base alloy of this invention contains from 6 to 12% chromium, from 2 to 6% iron, from 1 to 4% zirconium, and suflicient carbon and/ or nitrogen to constitute a zirconium to carbon atom ratio ranging from 1.5 :1 to 2:1, and up to 1500 parts per million of residual oxygen. Still another useful alloy of this invention having a vanadium base contains from 4 to 9% iron, from 4 to 8% columbium, from 1 to 4% zirconium, and sufficient carbon and/or nitrogen to constitute a zirconium to carbon and nitrogen atom ratio ranging from 1.5:1 to 2:1, and up to 1500 parts per million of residual oxygen. Finally, another useful alloy of this invention having a vanadium base contains from 6 to 12% chromium, from 6 to 12% tantalum, from 1 to 4% zirconium, sufficient carbon and/ or nitrogen to constitute a zirconium to carbon and nitrogen atom ratio ranging from 1.5 :l to 2:1, and up to 1500 parts per million of residual oxygen.

Alloys of the invention were prepared having the compositions set forth in Table I.

TAB LE I Zr N C V 1. 27 0. 023 0. 049 Balance. 1. 24 0. 021 0. 050 Do. 1. 3 0. 021 0. 053 Do. 1. 33 0. 022 0. 054 Do. 1. 30 0. 023 0. 054 Do. 1. 24 0. 021 0. 050 Do. 33 0. 023 0. 052 Do. 1 34 0. 024 0. 052 Do. 32 0. 051 D0. 1. 33 0. 062 Do. 1. 34 0. 052 D0. 1. 32 0. 053 Do. 1 34 0. 052 Do. 1 35 0. 055 Do. 1 34 O. 054 D0. 1. 34 0. 055 D0. 1. 33 0. 044 Do. 1. 30 0. 052 Do. 1. 24 0. 049 Do. 1. 33 0. 123 Do. Do. Do.

I All alloys contained about 0.1% oxygen.

2 About 0.02% nitrogen.

persed phase of fine particles of zirconium carbide, zirconium nitride, and zirconium oxide. The zirconium also acts as an oxygen getter for increasing corrosion resistance at elevated temperatures. Without zirconium and/ or titanium or hafnium being present, the carbon, oxygen, and nitrogen would combine with vanadium interstitially that would decrease fabricability of the vanadium.

One useful group of alloys of this invention contain the foregoing elements in the ranges of, by weight, from 5 to 12% columbium (niobium), from 1 to 4% zirconium, carbon, and nitrogen to constitute a zirconium to carbon and nitrogen atom ratio ranging from 1 /2:1 to 2:1, and up to 1500 parts per million of residual oxygen. Another useful alloy having a vanadium base contains elements in the range of, by weight, from 6 to 12% chromium, from 6 to 12% tungsten, from 1 to 4% zirconium, suflicient carbon and/0r nitrogen to constitute a zirconium to carbon and nitrogen atom ratio ranging from 1 /2 :1 to 2:1, and up to 1500 parts per million of residual oxygen. Another useful alloy of this invention having a vanadium base contains from 6 to 12% chromium, 3 to 9% molybdenum, from 1 to 4% zirconium, suflicient carbon and nitrogen to constitute a zirconium to carbon and nitrogen atom ratio ranging from 1 /2 to 2, and up to 1500 parts per million of residual oxygen.

Ordinarily, the nitrogen content of the alloys is residual nitrogen present in the alloy constituents or introduced during melting. During melting, an analysis is made to determine the nitrogen and carbon present and additional carbon is then added in an amount to make up the indicated 1 /2 :1 to 2:1 atom ratio of zirconium to total carbon and nitrogen.

The Table I also includes two meritorious vanadium base high titanium alloys for comparative purposes; these alloys have attractive tensile properties at elevated temperatures. The quantities in Table I are expressed in weight percent.

Each of the exemplary alloys A to W of Table I has a substantial merit in its own right. Each of these alloys is representative of a difierent composition within the broad invention.

Each of the alloys in Table I was worked into a sheet and after precipitation heat treatment was subjected to creep tests at 700 C. and at 800 0., similar tests being also performed on the binary vanadium titanium alloys. The results of these tests are presented in Table II.

The alloys used in Tables I and II were prepared by double arc melting of consumable electrodes in a partial pressure of atmosphere of argon. However, the ingots of the alloys may also be prepared by other means such as non-consumable arc melting of electron beam melting techniques.

Test specimens for the alloys may be prepared in a number of ways. A typical method used here for all the alloys was to initially hot work an ingot of from 1.5 to 2.5 inches diameter by forging to effect a 50% reduction at about 1000" C. The specimen was then rolled at about 500 C. to effect a 50% to 75% reduction and then cold work the specimen to final size of 0.042 inch sheet. The test specimens were then heat treated above 1200 C. for about one hour to produce a relatively coarse grain size of 3 to 5 ASTM.

TABLE II.CREEP PROPERTIES OF VANADIUM ALLOYS Minimum Tensile strength (p.s.i.)

Yield strength Elong., (p.s.i.)

percent Stress (p.s.i.)

1 Stress increased at 40,000 p.s.i. during test.

2 Test stopped.

3 Ruptured.

4 Heat treated.

As indicated in Table II, the alloys of this invention have superior creep resistance properties compared to those of the best known binary vanadium titanium alloys under similar test conditions.

The results of the tests for time to strain at /2%, 1%, and 3% are shown for 700 C. and 800 C. in FIGS. 1 and 2, respectively, from which the improved physical properties of alloys A to W as compared to X and Y are clearly evident. As shown in FIGS. 1 and 2 as well as Table 11, all of the alloys A, B, C, D, E, F, G, and H exhibit noted improvements in resistance to creep deformation over the binary vanadium titanium alloys X and Y. The creep rate improvement is particularly noted in alloys E and F. The high creep resistance exhibited by these alloys is attributable in part to the dispersed second phase faces of the carbides and nitrides of zirconium which inhibit dislocation motion and grain boundary movement and thereby decreases the creep rate. Moreover, the alloys of this invention contain relatively low substitutional solutes as compared with the binary vanadium titanium alloys. The amounts of substitutional solute elements present in the alloys is an important consideration because fabricability is influenced by the amount of such solid solution strengtheners present. Nuclear characteristics (i.e. breeding rating, etc. are degraded by large additions of substitutional solutes having high nuclear cross-section.

The alloys of the present invention combine solid solution and dispersed phase strengthening to achieve high strength and high temperatures. Vanadium base alloys which have been previously commercially available contain a high level of interstitials which tend to seriously degrade their fabricability. In the alloys of the present invention, zirconium and titanium combine with the interstitals carbon, oxygen, and nitrogen to form stable carbides, oxides, and nitrides which form the basis of the dispersed strengthening. Prior to precipitation hardening, the zirconium, for example, does not adversely affect fabricability. The addition of zirconium permits oxygen levels of 1500 parts per million in the 'vanadium base materials without sacrificing fabricability. Sufiicient interstitial carbon is added to the base alloy in order to attain a sufiicient volume fraction of dispersoid which contributes to the creep resistance of the alloy but does not reduce fabricability prior to precipitation hardening.

creep rate percent/ hour Time at temp. (hour) Time to strain, hours at- Strain, 5% percent A slight excess of zirconium or titanium may be used in the alloys. Moreover, because of the righ reactivity of zirconium or titanium, some scavenging of oxygen occurs. Corrosion tests in liquid sodium on these alloys have about half the weight gain of the binary ivanadium titanium reference alloys in similar tests. Apparently, a tenacious surface layer is formed on the alloys of this invention which reduces the diffusion rate of oxygen into the alloy body.

Alloys can be wrought into bolts, rods, tubes, and sheets for use in elevated temperature conditions such as in a fast breeder reactor.

It will be understood by those skilled in the art that although the invention has been described in connection with preferred alloys, modifications and variations may be employed without departing from the underlying spirit and scope of the invention.

What is claimed is:

1. A vanadium-base alloy consisting essentially of, by weight, from about 1% to 4% zirconium, from 3% to 20% of at least one element selected from the group consisting of columbium, chromium, tungsten, molybdenum, iron, nickel, titanium, and tantalum, from 0.02% to 0.1% carbon, from 0.001% to 0.05% nitrogen, the atomic ratio of zirconium to total carbon and nitrogen ranging from about 1.521 to 2:1, up to 0.15% oxygen, and the balance being essentially vanadium.

2. The alloy of claim 1, in the heat-treated, precipitation hardened state with an average grain of from 3 to 5 ASTM, whereby the alloy has a high resistance to creep deformation at elevated temperatures.

3. The alloy of claim 1, wherein columbium when present amounting to from 4% to 12%, tungsten when present amounting to from 6% to 12% and being present with from 6% to 12 chromium, molybdenum when present amount to from 3% to 9% and being present with from 6% to 12% chromium, iron when present amonting to 2% to 6% and being present with from 6% to 12% chromium, nickel when present amounting to from 4% to 3% and being present with from 4% to 8% iron, and tantalum when present amounting up to 12% and being present with 6% to 12% chromium.

4. The alloy of claim 3 comprising about 10% columbium.

5. The alloy of claim 3 comprising about 8.48% chromium, and from about 6% to 12% tungsten.

6. The alloy of claim comprising about tungsten.

7. The alloy of claim 3 comprising about 9% chromium, and from about 3% to 9% molybdenum.

8. The alloy of claim 7 comprising about 5.48% molybdenum.

9. The alloy of claim 3 comprising about 9% chromium, and from about 2% to 6% iron.

10. The alloy of claim 3 comprising at least one element selected from the group consisting of to 3% nickel, 4 to 8% columbium, and about 6.5% iron.

11. The alloy of claim 10 comprising about 1.7% nickel.

12. The alloy of claim 10 comprising about 5.3% columbium.

13. The alloy of claim 3 comprising about 8.49% chromium, and from about 6% to 12% tantalum.

14. The alloy of claim 13 comprising about 9.85% tantalum.

15. A Wrought member of an alloy consisting essentially of by weight, from about 1% to 4% zirconium, from 3% to 20% of at least one element selected from the group consisting of columbium, chromium, tungsten, molybdenum, iron, nickel, titanium, and tantalum, from 00.2% to 0.1% carbon, from 0.001% to 0.05% nitrogen the atomic ratio of zirconium to total carbon and nitrogen ranging from about 1.5:1 to 2:1, up to 0.15% oxygen, and the balance being essentially vanadium.

References Cited UNITED STATES PATENTS 1,715,867 6/1929 Saklatwalla 134 2,805,153 9/1957 Rostoker 75-134 3,293,741 12/1966 Gilliland 75134 RICHARD O. DEAN, Primary Examiner 

